25 January 2020 | New Scientist | 35individuals, either at the same time or one
after another. In 2012, Benjamin Lahey at
the University of Chicago and his colleagues
analysed information on such diagnoses
among 30,000 people studied over three
years. Using factor analysis, they found that
the observed patterns of illness were best
explained by a general tendency towards
mental health conditions.
The following year, Avshalom Caspi and
Terrie Moffitt at King’s College London got
the same result. Their study used information
from 1000 people whose health had been
tracked for four decades since their birth in
the early 1970s. It was Moffitt and Caspi who
coined the term p factor to describe an
individual’s broad susceptibility to mental
health problems. “Once you have any given
mental disorder, it increases the likelihood
that you’ll have multiple other kinds of
disorders,” says Caspi.Puzzling heritability
The p factor can also explain puzzling patterns
of mental health conditions within families.
It had long been known that these conditions
have a genetic basis, and are highly heritable.
Huge twin studies have estimated the
heritability of schizophrenia, for example,
at nearly 80 per cent, and major depression
at about 45 per cent. But having a parent or
sibling diagnosed with a given condition
doesn’t just increase the odds that you will
experience it. It also increases the likelihood
that you will be diagnosed with a different
condition. For instance, if a parent has
schizophrenia, your risk of developing
bipolar disorder doubles, and vice versa.
That makes sense if you inherit not just a
risk for one kind of condition, but a more
generalised risk: the p factor.
Indeed, the application of genetics to
psychiatry in the past decade has provided
key support for the existence of the p factor.
In the early days, psychiatric genetics mostly
entailed a hunt for individual genes conferring
significant risk for developing certain
conditions. But this so-called candidate gene
approach hit the skids. “It was really a dead
loss, but it was all we could do at the time,”
says Plomin. “Then SNP chips came along in
the mid-2000s and changed everything.”
SNP (pronounced “snip”) chips, which look
a bit like the memory card in a digital camera,
allow scientists to use a small DNA sample to
scan someone’s genome and discover which
genetic variants they carry. Everyone has
millions of single-letter differences in DNA’s >Sign up for our new health newsletter!
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